NEARBED SEDIMENT CONCENTRATION FROM TRACER STUDIES

Abstract

This paper reports on field investigations, in which the nearbed layer was one of major foci. The study was carried out at the IBW PAN Coastal Research Facility Lubiatowo. The radioactive sand tracer used in the study consisted of iridium glass beads with medium grain diameter D50 from 0.2 to 0.25 mm and having physical properties close to their counterparts of natural sand on the Baltic coast of Lubiatowo. Some 150 core samples, each 30 cm long, were taken from the areas where the tracer was set in motion, i.e. bar crests, troughs between bars, and other regions subject to the action of waves and currents. Data analysis shows that sediment movement is characterized by high intermittence reflected in a stratified structure of bedlayer, whereby each sublayer is governed by different forcing factors. For identical external conditions the vertical tracer profiles are different at bar crests and troughs, showing clear diversification of sediment mechanics across the surf zone. Fairly thin sediment sheets, a few millimeters thick, travel at bar crests and troughs, due to high sand speed in onedirectional (longshore) movement. The aggregated vertical profile of tracer concentration stems from the superposition of a few quasi-parabolas (close to the injection point) or thin uniform laminas (firmer away from the tracer source).